Feline Dirofilaria Immitis (Heartworm) Antibody Test (IFA)
Feline Dirofilaria immitis (FDI/FHW) otherwise known as Heartworm is a parasitic roundworm that is spread from host to host through the bites of mosquitoes. The heartworm is a type of filaria, a small thread-like worm, that causes filariasis. Cats can be infected, even though the definitive host is the dog. It can also infect wolves, coyotes, foxes and other animals, such as ferrets, sea lions, and even, under very rare circumstances, humans. The parasite is commonly called “heartworm”; however, adults often reside in the pulmonary arterial system (lung tissues). Occasionally, adult heartworms migrate to the right heart and even the great veins in heavy infections. Heartworm infection may result in serious disease for the host, with death typically as the result of congestive heart failure.
Heartworms go throughout several life stages before they become adults infecting the pulmonary artery of the host animal. The worms require the mosquito as an intermediate stage to complete their life cycles. The rate of development in the mosquito is temperature-dependent, requiring about two weeks of temperature at or above 27°C (80°F). Below a threshold temperature of 14°C (57°F), development cannot occur, and the cycle will be halted. As a result, transmission is limited to warm months, and duration of the transmission season varies geographically.
The period between the initial infection when the cat is bitten by a mosquito and maturation of the worms into adults living in the heart takes six to seven months and is known as the “prepatent period”. After infection, the third-stage larval heartworms (L3) deposited by the mosquito grow for a week or two and molt to the fourth larval stage (L4) under the skin at the site of the mosquito bite. Then, they migrate to the muscles of the chest and abdomen, and 45 to 60 days after infection, molt to the fifth stage (L5, immature adult). Between 75 and 120 days after infection, these immature heartworms then enter the bloodstream and are carried through the heart to reside in the pulmonary artery. Over the next three to four months, they increase greatly in size. The female adult worm is about 30 cm in length, and the male is about 23 cm, with a coiled tail. By seven months after infection, the adult worms have mated and the females begin giving birth to live young, called microfilaria.
The microfilaria circulate in the bloodstream for as long as two years, waiting for the next stage in their life cycles in the gut of a bloodsucking mosquito. When ingested by a mosquito the microfilaria undergo a series of molts to the infective third larval stage, and then migrate to the salivary glands of the mosquito, where they wait to infect another host. The incubation period required to reach the stage where the microfilaria become transmittable to another host can be as little as two weeks or as long as six weeks, depending on the warmth of the climate, and the larval life cycle ceases entirely if the ambient temperature drops below 14°C (57°F).
Cats show no indication of heartworm infection during the six month prepatent period prior to the worms' maturation, and current diagnostic tests for the presence of microfilaria or antigens cannot detect prepatent infections. However, antibodies to larval stages can be detected by use of an indirect fluorescent antibody (IFA) test done on fixed microfilaria. This test will give an indication of exposure and possible prepatent infection as early as 3 weeks post infection. Rarely, migrating heartworm larvae get “lost” and end up in unusual sites, such as the eye, brain, or an artery in the leg, which results in unusual symptoms such as blindness, seizures and lameness. But normally, until the larvae mature and congregate inside the heart, they produce no symptoms or signs of illness. Many cats will show little or no sign of infection even after the worms become adults.